1. Field of the Invention
The invention relates to a parametrical generation laser and more particularly to a semiconductor laser used for parametrical generation from a pump wave xcfx891.
Parametrical fluorescence is indeed well known in non-linear optics. This phenomenon is the generation, within what is called a non-linear material and from a beam known as a pump beam (at the frequency referenced xcfx891), of two beams (known as the signal and idler beam at the frequencies xcfx891 and xcfx893). The principle of the conservation of energy implies xcfx892+xcfx893=xcfx891. By placing the non-linear material in a cavity in which one or two generated frequencies resonate, it is possible to obtain an optical parametrical oscillator (OPO). OPOs are very widespread, and are used as light-tuneable coherent sources at frequencies that are poorly covered by lasers (the mean infrared or visible range for example). The problem with OPOs is their high space requirement and complexity since the full system comprises a pump laser, a non-linear crystal and cavity mirrors.
2. Description of the Prior Art
The difficulty of obtaining parametrical generation is related to the need for what is known by those skilled in the art as phase matching. Owing to the dispersion of the optical index with the wavelength, the different interacting waves (xcfx891; xcfx892 and xcfx893) do not move at the same speed in the material. The result of this is that non-linear interaction becomes highly destructive and the process loses its efficiency. To obtain efficient parametrical generation, it is therefore necessary to obtain phase matching which maintains a constructive interaction throughout the propagation. This phase matching, which can also be interpreted as a conservation of moment, can be written as the relationship between the optical indices n1 at the frequencies xcfx891:
n2xcfx892+n3xcfx893=n1xcfx891.xe2x80x83xe2x80x83(1)
In OPOs made until now, different techniques are used to meet this indispensable relationship: for example phase matching using birefringent materials or again quasi-phase matching are the two most commonly used techniques. Another method is a modal phase matching. It consists in using the dispersion relationship of the different modes existing inside a waveguide to finally verify the relationship (1). This is impossible if we consider only the fundamental mode of the waveguide. It is therefore necessary to use different order modes for the different waves. This method of modal phase matching is the one used in the framework of the invention. The invention uses for example the fundamental mode for the signal and idler waves but the second-order mode for the pump wave. For this method, it is shown that the efficiency of the parametrical generation is proportional to the following integral:
∫∫dxdy"khgr"(2)(x,y)Excfx8912*Excfx8921Excfx8931(x,y)xe2x80x83xe2x80x83(2)
The integral is obtained in the plane perpendicular to the waveguide (section plane) and Excfx8911 designates the field at the frequency xcfx89j for the i order mode. This is a major difficulty for preventing the non-nullity of this integral once we take account of different order modes. Indeed, the relationship of orthogonality between the different order modes cancels the integral in principle:
∫∫dxdyExcfx8912Excfx8911xe2x80x83xe2x80x83(3)
Since the fundamental modes Excfx8911 and Excfx8921 at the frequencies xcfx891 and xcfx892 are, in a usual waveguide, quasi-identical, the integral (2) is always small. This has hitherto limited the use of this modal phase matching method According to the invention, the device is designed precisely so that the waveguide has a big integral (2). The conclusion here is that, for the parametrical generation in a waveguide to be efficient, the relationship (1) (phase matching) should be verified and the overlapping integral (2) should be optimized.
The invention therefore relates to a laser structure in which the laser wave may be used as a pump source for parametrical generation, this laser structure being designed to verify these two conditions.
The invention thereof relates to a semiconductor laser comprising at least two layers of optically non-linear material as well as a quantum well at least located within one of the layers of optically non-linear material, the thicknesses and optical indices of these two layers being such that the waveguide constituted by these two layers has a modal phase matching condition for the process of parametrical fluorescence between the pump wave emitted by the quantum well and the parametrical conversion waves.